A solder resist is used in soldering parts to a printed circuit board for prevention of adhesion of the solder to unnecessary parts and for protection of the circuit. It is required to have adhesion to a substrate, electrical insulating properties, resistance to soldering temperatures, solvent resistance, alkali resistance, acid resistance, and resistance to plating.
Epoxy-melamine type heat-curing resins, which were used as solder resists in early years, are unsatisfactory in heat resistance to soldering, chemical resistance, and resistance to plating. Improved epoxy resin type heat-curing solder resists have been proposed for industrial use for the production of printed circuit boards as disclosed in JP-B-51-14044 (the term "JP-B" as used herein means an "examined Japanese patent publication") and are currently widespread. For domestic use where weight is given to productivity, rapid-setting ultraviolet-curable solder resists as disclosed in JP-B-61-48800 are widely employed. However, UV-curing solder resists, when applied thick, show insufficient curing in the inside and also have poor resistance to soldering temperatures and are therefore unsuitable for use in industrial production of printed circuit boards.
On the other hand, with the latest demands for weight and size reduction of electronic parts, printed circuit boards have been increasing the circuit density, and surface mounting of parts on printed circuit boards have been advanced. Under such a situation, while solder resist patterns are generally formed by screen printing, these conventional solder resist compositions when screen printed find difficulty in avoiding bleeding and in sufficiently embedding among circuit pattern lines and can no longer perform their function as a solder resist film.
In order to overcome the above problem, dry film type photosolder resists and liquid photosolder resists have been developed.
Dry film type solder resists include, for example, photosensitive resin compositions containing urethane di(meth)acrylate, a cyclic high polymeric compound having a specific glass transition point, and a sensitizer as disclosed in JP-A-57-55914 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, when applied to a high-density printed circuit board, these dry film type photosolder resists have insufficient heat resistance in soldering and insufficient adhesion.
Known liquid photosolder resists include a photopolymerizable coating composition containing a solid or semi-solid reaction product between polyepoxide and an ethylenically unsaturated carboxylic acid, an inert inorganic filler, a photopolymerization initiator, and a volatile organic solvent as disclosed in British Patent (Publication) 2032939A. However, since the composition relies only on a UV-curing component without utilizing heat curing, the resulting resist film is insufficient in adhesion, heat resistance in soldering and electrical insulating properties.
Taking heat curing in combination with UV curing into consideration, JP-B-1-22312 discloses a resin composition for solder resist ink comprising a reaction product between a phenol-novolak epoxy resin and an unsaturated monobasic acid, a partial reaction product between a cresol-novolak epoxy resin and an unsaturated monobasic acid, an organic solvent, a photopolymerization initiator, and an amine type curing agent. In this case, curing is achieved by UV curing in combination with heat curing due to residual epoxy groups in the molecule. However, as the residual epoxy group increases in content, the photosensitive group relatively decreases in content so that the composition has reduced UV curability, and it is difficult to leave a sufficient amount of epoxy groups for satisfying the characteristics demanded as a solder resist. In addition, the use of a halogen type organic solvent for development is unfavorable from the consideration of the working environment.
JP-B-1-54390 discloses a photosolder resist composition capable of development with an alkali aqueous solution which comprises a reaction product among a novolak epoxy resin, an unsaturated monobasic acid and a polybasic acid anhydride, a polyfunctional epoxy resin, a diluent, and a photopolymerization initiator. In this case, too, an increase of the proportion of the epoxy resin for improvement of resist characteristics necessarily leads to a reduction of the photosensitive component, resulting in reduction of photosensitivity. As a result, the coating film is liable to be corroded during development only to provide low resolution. Thus, this technique still fails to satisfy both photosensitivity and other characteristics as a solder resist. Moreover, the composition has poor preservation stability because the epoxy resin gradually undergoes a reaction with the carboxyl groups in the photosensitive oligomer. To avoid this, the composition is usually provided as a two-pack type composition, requiring mixing of two liquids on use.
JP-B-1-54390 discloses a photo- and heat-setting liquid resist ink composition capable of development with a dilute alkali solution which comprises a resin obtained by reacting a reaction product between a novolak epoxy compound and an unsaturated carboxylic acid with a saturated or unsaturated polybasic acid anhydride, a photopolymerization initiator, a diluent, and an epoxy compound having two or more epoxy groups in the molecule. The composition is made alkali-developable by the carboxyl groups of the photosensitive resin. The epoxy groups of the di- or polyfunctional epoxy compound as a heat-curing component undergo copolymerization with free carboxyl groups to contribute to manifestation of various properties as a coating film. However, existence of the di- or polyfunctional epoxy group induces a phenomenon of insufficient development called heat fog. Besides, incorporation of the epoxy compound decreases the proportion of the photosensitive component and thereby reduces photo-curability, which leads to deterioration in sensitivity and resolution.